Detectors of ionizing radiation such as gamma and x-ray radiation quanta that are used in for example PET or x-ray imaging systems conventionally include a scintillator element and a photodetector. The scintillator element receives the radiation quanta, and converts each radiation quantum into a pulse of infrared, visible or ultraviolet light that is detected by the photodetector. The resulting electrical pulse from the photodetector is subsequently analyzed to determine characteristics of the radiation quantum.
In a PET imaging system the time of reception and the energy of each radiation quantum are used respectively to determine and validate the origin of radioactive decay events. Gamma photons that are received within a narrow predetermined time interval of each other by detectors disposed around a PET imaging region are indicative of their generation at a common origin, and gamma photons having energies within a predetermined narrow range are indicative of the absence of path-altering scattering between their points of origin and detection. In a PET imaging system the time of reception of a gamma photon is determined by a timestamping unit which records the time at which the photodetector's electrical signal exceeds a predetermined threshold. A coincidence determination unit subsequently identifies pairs of timestamps that occur within a narrow time interval, typically within +/−5 ns of each other, as coincident events. The energy of each gamma photon is determined by integrating the photodetector's electrical signal; thus by summing the energy from the individual optical photons produced in the scintillator element by the gamma photon.
The advantage of such a scintillator-based detector in which a photodetector generates the timing signal is its fast response. Currently-used scintillator materials such as LYSO (Lu,Y)2SiO5:Ce and GAGG Gd3(Al,Ga)5O12:Ce are capable of generating a timestamp with an accuracy of approximately a few hundred picoseconds, making them suitable for use in PET imaging applications. The fast decay times of LYSO and GAGG of approximately 45 ns and 90 ns respectively contribute to this timing accuracy by ensuring that the scintillation light decays to a negligible level between consecutively-received gamma photons. However, the energy discrimination of such scintillator-based detectors is hampered by the relatively low light yield of scintillator materials. LYSO has a light yield of approximately 32000 photons/MeV, and GAGG has a light yield of approximately 65000 photons/MeV. At these light yields the photon statistics limit the energy resolution to values of 10-12% for 511 keV gamma photons.
Electrical signals from a photodetector in a scintillation-based x-ray detector are generated in an analogous way. Scintillator materials such as Gd2O2S doped with Pr (GOS) and (Y,Gd)2O3 doped with Eu that are typically used in x-ray CT are however too slow to provide timing information for individual x-ray quanta at the radiation flux densities used in CT imaging. Consequently when these materials are used in CT the photodetector's electrical signal is integrated in order to determine the received x-ray flux density. When spectral CT is implemented with such materials, energy discrimination is provided either by kV switching in which the radiation source is temporally switched to generate x-ray radiation quanta at different energies, or by generating x-ray radiation quanta with different energies simultaneously and using a stacked detector to discriminate the energy of each radiation quanta based on its absorption depth in the detector.
Photon-counting x-ray detectors have also been used in the field of spectral CT in which materials such as CZT ((Cd,Zn)Te) directly convert the energy of a received x-ray radiation quantum to a charge signal. When the charge signal exceeds a threshold it triggers a counter that records the total number of radiation quanta traversing a particular line in space. Such a configuration may be used to determine the attenuation of matter between an x-ray source and detector. Furthermore, the amplitude of each individual electrical signal is indicative of the energy of the quantum, permitting energy discrimination of the received quanta. By comparing the counts at different quantum energies for a particular line in space, further properties of the intervening matter may be determined. Such direct-detection x-ray detectors however have inherently poor timing accuracy. The drift time of the charge cloud generated in response to a received x-ray quantum takes some 100 ns to reach the detector's contacts where it is detected. However, in x-ray detection the absolute time of the reception of each radiation quantum is of minor importance, so inherent variations in the time taken for the charge cloud to drift to the detector's contacts have little significance. Such a direct-detection technique capable of counting and discriminating based on quantum's energy therefore finds application in spectral CT imaging. By contrast, in PET imaging such variability in the timing signal would be unacceptable, restricting the application of this technique to x-ray detection.
A document “Investigation of liquid xenon detectors for PET: Simultaneous reconstruction of light and charge signals from 511 keV photons” by P. Amaudruz et al., Nuclear Science Symposium Conference Record, 2007. NSS 07. IEEE, vol 4, pp. 2889-2891 discusses another scintillation-based gamma photon detector in which liquid xenon is used as the scintillator element. A photodiode coupled to the container of the liquid xenon detects the optical signal generated in response to a received gamma photon, and electrodes disposed on the surfaces of the container are electrically biased so as to separate ionization charge carriers that are generated by the gamma photon. Wires that are disposed in the liquid xenon measure the current induced by electrons as they drift between the electrodes in a configuration known as a time projection chamber. An energy resolution of less than 4% was achieved by combining scintillation light an ionization charge.
However, the drawbacks of using liquid xenon in such scintillation-based detectors are several, including the need for cooling or high pressure containment. Furthermore, with a density of 2.978 g/cm3 at the triple point temperature of 161.4 K, in order to capture the same proportion of incident gamma photons, liquid xenon requires a thicker scintillator element than traditional scintillator materials such as LYSO which has a density of 7.3 g/cm3. The safety issues associated with use of large volumes of cooled and or high pressure liquid xenon further complicate the practicality of its use in for example a PET imaging system.
Consequently a need remains for gamma photon and x-ray detectors in which good timing accuracy and good energy resolution may be obtained in the absence of the drawbacks of such practical and safety issues.